The present invention relates to a system for controlling hydraulic fluid pressure for an automatic transmission.
Laid-open Japanese Patent Application No. 60-249758 discloses an automatic transmission. This known transmission includes a gear power train and a V-belt type continuously variable power train. Power is transmitted via one of the above-mentioned two power trains in response to operating condition. When a large driving force is required, the gear power train is rendered operative to transmit power between the input and output shafts, while when a small driving force is required, the V-belt type continuously variable power train is rendered operative to transmit power between the input and output shafts.
In the known transmission, upon transmission of power via the gear power train, driver and follower pulleys and a V-belt drivingly connected between the pulleys rotate. Thus, there occurs frictional loss not only in the gear power train, but also in the V-belt type continuously variable power train, causing a drop in power transmission efficiency. Further, hydraulic fluid pressure is kept applied to cylinder chambers of the pulleys, inducing a loss on pump used as a source of pressurized hydraulic fluid. Besides, since the V-belt is always subject to tension, the durability of the V-belt and pulleys drops.
An object of the present invention is to solve the above-mentioned problems.